This invention relates generally to a method of increasing the corrosion resistance of pre-treated steel, and more particularly, to a method of treating the interior enamel of steel containers suitable for packing food.
Many types of corrosion resistant materials are utilized for coating steel. Alkali metal silicate is used to coat metal and glass as described in U.S. Pat. No. 3,526,530. U.S. Pat. No. 3,730,746 shows that preparation of a silicate polymer as a protective coating for metal. A method of coating a corrosible steel substrate having an oxidized steel coating with a solution of an alkyl silicate is disclosed in U.S. Pat. No. 4,071,380. The alkyl silicate coating is allowed to cure before a top coating is applied. This process insures the maintenance of the oxidized steel coating as an integral component in a system for inhibiting corrosion. The porous character of a fully cured or hydrolyzed alkyl silicate is called upon to provide passage for oxygen and water for effecting corrosion inhibiting oxidizing action with the underlining steel substrate, with the resultant oxidized steel being retained within the pores of the inorganic cured solution as an integral part of the film. Additionally, the alkyl silicate may function as a vehicle or binder for a mixture of dry, inert particulate matter which facilitates the silicate application of the solution by its particular color and aids the appearance of the subsequently applied top coat by smoothing the surface. U.S. Pat. No. 3,983,305 discloses a process for treating cold, rolled steel having a residual oil coating in a manner to provide an effective binding substrate for an organic protective coating. The surface of cold rolled steel has a relatively low corrosion resistance and relatively poor organic coating characteristics. Therefore, it is coated with oil and subjected to heat treatment until the corrosion resistance and the organic coating characteristics of the resultant heat treated steel are improved. The heat treated steel is then cooled to prevent further thermal degradation of the rolling oil to produce a residual film on the heat treated steel surface. In conventional methods, many steps are required before applying an organic protective coating: efficient cleaning, heating to decompose the oil, removal of the residual oil or other contaminants, and then applying a temporary protective coating.
In the food industry, the interior of the enamel coated steel containers is attacked by the food product resulting in the oxidation of the steel container and/or the formation of salt deposits which discolors the steel, and are clearly visible to the consumer. There is a need for further protecting the steel after conventional protective coatings have been applied to improve the corrosion resistance characteristics of the steel.